1. Field of the Invention
The present invention relates to an input-output balanced filter, and more particularly, to input-output balanced filters incorporated in communication equipment or the like, such as cellular phones and automobile phones, etc.
2. Description of the Related Art
In general, communication equipment such as a cellular phone, an automobile phone, and the like, use a differential filter or an input-output balanced filter adapted to function both as a filter and an impedance transformer, arranged between the stages of a mixer and a modulator in a transmitting circuit. As a conventional type of input-output balanced filter, a filter shown in the current equivalent circuit diagram of FIG. 13 is known. The filter 101 has a circuit structure in which an LC parallel resonant circuit 102 including an inductor L21 and a capacitor C21, and an LC parallel resonant circuit 103 including an inductor L22 and a capacitor C22 are connected via connecting capacitors C25 and C26. In the filter 101, the signal input between input terminals 111a and 112a is filtered and then experiences impedance transformation so as to be output between output terminals 111b and 112b. 
Conventionally, in forming an input-output balanced filter 101, an inductor and a capacitor, which are discrete components, are arranged to be connected via a circuit pattern disposed on a printed circuit board or the like. This causes the size (the occupied area) of the filter 101 to be greatly increased, thereby preventing reduction in size or miniaturization of the device. Another more serious disadvantage is that variations in respective electric constants of the discrete components cause a balanced transmission characteristic of the input-output balanced filter 101 to deteriorate significantly. Also, since the electric constants of the discrete components are usually set by ranking of specified values, it is difficult to make fine adjustments to the electric constants of the components so that a desirable characteristic can be obtained. Another problem is that there are fluctuations in the characteristic of the filter 101 depending on the condition in which the discrete components are mounted.